Using the resources of an epidemiologically defined, longitudinal twin study, we examine developmental course and biopsychosocial risks for childhood psychopathology. Building on existing screening at ages 2 and 7 years, we characterize risk factors at ages 2, 7, and 12 relevant to the development of internalizing (anxiety and depression), externalizing (oppositional and conduct disorder), and ADHD. Most of our at-risk cases qualify for a DSM-IV diagnosis, yet we include individuals with sub-threshold symptoms and a large control group. Specific aims include characterization of risk factors, analysis of genetic and environmental effects, and improved measurement and classification of childhood disorders. "Child-based" risk factors include earlier symptoms, temperament and stress reactivity, basal and reactive cortisol, testosterone and DHEA, cognitive abilities and attributional styles, and cognitive-affective processing skills. Family and other psychosocial risk factors include parental diagnosis and family history of psychopathology, twin-twin and twin-parent social interaction styles, multiple facets of family stress, and negative parenting. The research methods that we employ include structured diagnostic interviews with caregivers and children, medical records, observer ratings, child self-report and parent-report questionnaires, videotaped home-based behavioral batteries, and computer-based testing (mostly reaction time tasks). The study's significance lies in understanding how known risk factors interact and/or mediate each other's effects on child psychopathology in a genetically informative, longitudinal design. Twin methodology allows us to parse phenotypic variance and covariance among measures into genetic and environmental components, and the components are studied developmentally. The results should also enhance our understanding of comorbidity, heterogeneity within disorders, and the association of disorders with traits. The project's public health relevance lies in its identification of risk factors for common childhood disorders and its investigation of how they interact in the context of genetic and environmental factors. The knowledge gained should inform efforts toward early detection, improved interventions, and better classification of childhood disorders.